The present invention generally relates to a tuner for signal reception, and more particularly, to a double super-heterodyne tuner suitable for receiving television broadcasting signals and CATV (cable television) signals, etc.
It has been a recent trend that multi-channel signal reception has been widely adopted, for example, as in CATV, and double super-heterodyne tuners which are advantageous for multi-channel signal reception are being introduced for the purpose.
Hereinafter, a conventional double super-heterodyne tuner will be briefly explained with reference to FIGS. 1 to 3.
In FIG. 1, there is shown a block diagram of the known double super-heterodyne tuner system, which includes input filters 1, 2 and 3 respectively having different band-pass regions and sequentially coupled with an AGC (Automatic gain control) circuit 4 constituted by a pin attenuator, etc., an RF amplifier 5, a first mixer 6 connected to a first local oscillator 7 having a variable oscillating frequency, a first IF amplifier 8, a first IF band-pass filter 9, a second mixer 10 connected to a second local oscillator 11 having a fixed oscillating frequency, and a second IF amplifier 12 connected to an output terminal B as illustrated.
In operation, the television signal inputted from a terminal A passes through one of the input filters 1 to 3 and is applied to the AGC circuit 4. The signal, gain controlled by the AGC circuit 4, is amplified at the broadband RF amplifier 5 and then inputted to the first mixer 6, to which an output signal of the first local oscillator 7 is also applied, with a difference signal between an output signal of the RF amplifier 5 and the output signal of the first local oscillator 7 being produced at an output of said first mixer 6. This difference signal is applied to the first IF amplifier 8, and the signal amplified thereat is filtered by the band-pass filter 9 to be further applied to the second mixer 10, where the signal is mixed with the signal of the second local oscillator 11, and the mixed output signal from the second mixer 10 is outputted from the terminal B as the second IF signal through the second IF amplifier 12.
Subsequently, the input filters 1 to 3 referred to above in FIG. 1 will be explained more in detail with reference to FIGS. 2 and 3.
FIG. 2 schematically shows one example of the input filter, in which a generally known Chebyshev type low pass filter is combined with a high-pass filter, and which is normally designed for an input and output impedance of 50.OMEGA..
FIG. 3 shows a circuit diagram in which three band-pass filters each as shown in FIG. 2 are combined with each other so as to be capable of being changed over therebetween.
As shown in FIG. 3, the circuit generally includes band-pass filters 20, 21 and 22, and input and output terminals F and G coupled with the band-pass filters 20, 21 and 22 through switching diodes 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 and 34 which are grounded via capacitors and resistors, with junctions between the switching diodes 24 and 25, 28 and 29, and 32 and 33 being connected to terminals C, D and E through corresponding resistors as illustrated.
In the circuit construction of FIG. 3, the television signal or CATV signal inputted from the input terminal F is selected for the filter to pass through by band-pass voltages applied to the terminals C, D and E. For example, in the case where the voltage for the terminal C is OV, and that for the terminals D and E is +BV, the diodes 23 and 26 are turned ON, while the diodes 24 and 25 are turned OFF, and the signal passes through the band-pass filter 20. Meanwhile, since the diodes 28, 29, 32 and 33 are turned ON, and the diodes 27, 30, 31 and 34 are turned OFF, the signal does not pass through the band-pass filters 21 and 22. The three band-pass filters 20, 21 and 22 are respectively separated in the pass-band regions, e.g. as in the VHF low band, VHF high band, and CATV band.
However, the conventional construction as described so far has problems in that the filters are complicated in structure as shown in FIGS. 2 and 3, with the change-over circuits being increased in number, while the AGC circuit is separately required, thus resulting in increased costs.